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Abstract:

Due to the extirpation of their natural predators, feral horse populations have expanded across the United States, necessitating
their management. Contraception of females (mares) with porcine zona pellucida (PZP) is a popular option; however, effects to
physiology and behavior can be substantial. On Shackleford Banks, North Carolina, USA, treated mares have exhibited cycling
during the non-breeding season and demonstrated decreased fidelity to the band stallion, but PZP’s long-term effects on mare
physiology and behavior remain largely unexplored. After the contraception program was suspended in this population, we
examined how prior exposure to varying levels of PZP treatment impacted (1) foaling probability and foaling dates (a proxy for
ovulatory cycling) from 2009 to 2014 and (2) mare fidelity to the band stallion and reproductive behavior during 2013 and
2015. Additionally, we evaluated the effects of time since the mares’ last treatment on these factors. Mares receiving any level
of prior PZP treatment were less likely to foal than were untreated mares. Among mares that received 1–3 PZP applications,
foaling probability increased with time since last treatment before declining, at ~6 years post-treatment. Mares that received
4+ applications did not exhibit a significant increase in foaling probability with time since last treatment. Moreover, previously
treated mares continued to conceive later than did untreated mares. Finally, mares previously receiving 4+ treatments changed
groups more often than did untreated mares, though reproductive behavior did not differ with contraception history. Our
results suggest that although PZP-induced subfertility and its associated behavioral effects can persist after the cessation of
treatment, these effects can be ameliorated for some factors with less intense treatment. Careful consideration to the frequency
of PZP treatment is important to maintaining more naturally functioning populations; the ability to manage populations adaptively may be compromised if females are kept subfertile for extended periods of time.